This invention relates to low temperature curable surface coating compositions. More particularly, it relates to a process for preparing improved curable coating compositions comprising an aminoplast, a hydroxy-copolymer and a hydroxy containing resin.
It is well known in the coatings art to prepare coating compositions by dissolving in organic solvents an alkoxymethyltriazine or alkoxymethylurea and a hydroxy functional resin to provide coatings which exhibit satisfactory hardness. Efforts are also being made to cure the coatings at low temperature to conserve energy and to utilize such coatings on substrates such as wood, paper, paper board and plastics that cannot survive high temperature cure. However, low temperature cure gives coatings that are soft, have poor water and chemical resistance and poor durability.
In solvent coating systems the addition of a styrene-allyl alcohol copolymer has been found to improve the properties of coatings cured at low temperatures. The styrene-allyl alcohol copolymer is typically added to the vehicle separately from the aminoplast crosslinker. Until now, no specific procedure has been taught for the addition of the styrene-allyl alcohol copolymer to the aminoplast crosslinker or the vehicle. It has been found that the surface coating compositions obtained by the procedure described below provide improved surface coatings which exhibit superior inter coat adhesion particularly in automotive applications, where more than one coat of paint may be applied to the automobile to effect a two-tone color coating, and superior flexibility and hardness, properties which are generally mutually exclusive.
The method we have discovered consists of adding styrene-allyl alcohol copolymer to hydroxy functional resins containing an aminoplast.
Our method is achieved by
(1) forming a concentrated solution of an aminoplast and a styrene-(meth)allyl alcohol in a solvent and
(2) adding to the solution a non-aqueous solution of one or more hydroxy functional resins and mixing to obtain a uniform solution.
Another aspect of our invention is directed to the improved coating compositions provided by our process and to substrates coated with such compositions.